This invention relates to an oximeter and a medical sensor therefor, said medical sensor comprising light emitting means for emitting light into the tissue of a patient, light receiving means for receiving the light transmitted through or reflected by said tissue, and connection means for the connection of said medical sensor to said oximeter.
Pulse oximetry is a non-invasive technique to evaluate the condition of a patient. Usually, a sensor or a probe comprises light emitting means such as light-emitting diodes (LEDs). Two or more of these LEDs with different wavelengths (e.g. red and infrared) may be used. The emitted light is directed into the tissue of the patient, and light receiving means such as photodiodes or phototransistors measure the amount of transmitted or reflected light. In the case of transmission measurement, the transmitter and receiver diodes are arranged opposite to each other with respect to the human tissue, whereas in the case of reflection measurement, they are arranged on the same side of the tissue.
The measured intensity can be used to calculate the oxygen saturation in the arterial blood of the patient if measured at at least two wavelengths. has been described in detail in a lot of former publications. A rather good breakdown of the theory is e.g. contained in European Patent Application EP-A-262 778.
Usually, the sensor detachably connected to the oximeter comprises at least two LEDs emitting light of a wavelength of e.g. 650 nm (Nanometers)--red--and 1000 nm--infrared. The intensity of the emitted light can be modulated by the oximeter in that the exciting current of the LEDs is varied. The photocurrent received by the receiving element is measured by the oximeter and used to calculate the oxygen saturation of the arterial blood.
It is important that the wavelength of the emitting diodes is well-known as the used wavelengths directly influence the results of the calculation. U.S. Pat. Nos. 4,621,643, 4,700,708 and 4,770,179 therefore propose to use an encoding element, usually a resistor, implemented in the sensor to encode the wavelengths. The oximeter decodes the value and selects appropriate extinction coefficients for oxygen saturation calculation.
It has been found that such encoding does, under some particular circumstances, not lead to sufficiently accurate measuring results. In particular, the oxygen saturation calculated by means of said extinctions coefficients according to Lambert-Beer's law is based on an ideal optical model and has to be corrected empirically, i.e. by comparative measurements with a lot of patients. The inventor has noted that major reasons that the ideal model does not comply with practical measurements are:
1. Light is scattered by blood corpuscles;
2. The medium (human tissue) has inhomogeneous characteristics;
3. different paths for the light beams emitted by the various light sources (the light-emitting diodes are not located at the same place, and light refraction is not identical);
4. Due to the pressure exerted by the sensor, venous pulsations of the tissue are also recorded;
5. The light-emitting diodes do not have exactly monochromatic characteristics.
The differences between the theoretically calculated oxygen saturation value and the real value are considerable, e.g. approx. factor 2 for an oxygen saturation of 50%. This problem affects the accuracy of an oximeter.